This longitudinal study examined the degree to which standardized measures of language and natural language samples predicted later language usage in a heterogeneous sample of children with autism spectrum disorder (ASD), and how this relationship is impacted by ASD severity and interventions. Participants with a diagnosis of ASD (N = 54, 41 males) completed standardized assessments of language and social functioning; natural language samples were transcribed from play-based interactions. Findings indicated that standardized language measures, natural language measures, and ADOS severity were each unique predictors of later lexical use. Intervention types also appeared to impact later language; in particular, participation in mainstream inclusion accounted for significant amounts of variance in children's mean length of utterance at T3.